3.2 Population assignment and structure
Pairwise Fst comparisons between
regional samples showed low to moderate values ranging from 0.05
to 0.21; all comparisons were significant (Fig. S2, Supporting
information). The highest Fst value was
recovered between Guatemala and Australia, and
lowest Fst values were obtained comparing Spain
and Greece (0.05) and their respective comparisons with Australia (0.08
and 0.09). South African specimens showed
moderate Fst values in pairwise comparisons with
all sites, although slightly higher with the Spanish and Greek (0.18 in
both cases) than with Neotropical and Australasian sampling sites.
DAPC detected an optimal genetic cluster of two (K =
2) followed by three (K = 3) based on the AIC value (Fig. S3A,
Supporting information). DAPC results for K= 2 identified two
clusters (South Africa and Brazil) versus the rest, i.e. Spain, Greece,
Guatemala and Australia (Fig. S4A, Supporting information). The DAPC
results for K = 3, with 16 principal components explaining 46.7%
of the variance, showed two separated clusters (South Africa and Brazil)
and the rest (Fig. 2A). Group assignment analysis
(compoplot function) supported these clusters with a high
probability of assignation (Fig. S4B, Supporting information).
Unexpectedly, the assignment plot placed one individual from SA (SA_8)
(Fig. S4B-C, supporting information) into the cluster composed of all
other populations, indicating the ancestral presence of the colonising
alleles in low frequency or potential back colonisation from the
colonised areas. Moreover, STRUCTURE analyses identified K =2 as
the most probable number of genetic clusters followed by K =3,
with some differences in the groupings as compared to the DAPC analyses
(Fig. S3C, Supporting information). For K =2, one cluster grouped
all sampling sites except South Africa, which showed some degree of
introgression (<20%) from another genetic cluster (dark blue
in Fig. 2 top). When considering K =3, Brazilian individuals
showed a small proportion of cluster assignation to a cluster also
present in the South African population (light blue in Fig. 2B bottom).
Further DAPC and STRUCTURE analyses were conducted, excluding samples
from South Africa (Fig. 3A-B) to explore the introduced range in more
detail. DAPC detected an optimal genetic cluster of three (K = 3)
(Fig. S3B, Supporting information) with Brazilian samples as the most
divergent population and some differentiation between Spain-Guatemala
versus Greece-Australia (Fig. 3A). STRUCTURE analyses for K =2
detected a genetic break between Brazil and the rest of populations; forK =3, apart from identifying Brazil as belonging to a different
genetic cluster, all individuals from Guatemala were assigned to a
distinct genetic cluster, and individuals from Greece and Australia were
mostly assigned to a separate cluster (with the exception of two
individuals from Greece), while most individuals from Spain had mixed
association with the former two clusters in roughly similar proportions
(Fig. 3B).
The co-ancestry matrix generated
by fineRADstructure (Fig. 4) showed that all individuals of the
respective locations clustered together, with the exception of European
samples from Spain and Greece, which appeared mixed in two different
groups. The most distinct group of samples was South Africa, showing the
highest values of co-ancestry, followed by Brazil. Both also showed
higher ancestry relationships compared to the remaining sampling
locations, i.e. they presented fewer allele differences among them
compared to the remaining sampling sites. All other samples formed a
cluster that could be subdivided into two subclusters: subcluster A,
including Australia and a mix of samples from Spain and Greece; and
subcluster B, including all samples from Guatemala and the remaining
Spanish and Greek samples. The samples from Europe (Spain and Greece)
showed strong evidence of heterogeneous gene flow (indistinguishable
genetic ancestry).